A magnetic recording medium having excellent S/N ratio among various magnetic characteristics has been recently demanded with the increase of various usages of a magnetic recording medium. This demand has become particularly strong with a improvement of a video recording tape and an audio recording tape.
A ferromagnetic fine powder is required to have excellent dispersibility and orientation to satisfy the above demand. Therefore, (1) making fine particles of a ferromagnetic powder, (2) adding a dispersing agent (such as a surface active agent), (3) adding a coagulation-preventing agent (such as solid particles) and (4) introducing a powerful dispersing machine have been proposed to improve the dispersibility of a ferromagnetic powder and decrease tape noise.
Even though high dispersibility of the coating composition may be achieved at one point, ferromagnetic fine powder readily coagulates while being dried, and high dispersibility cannot be retained.
Making a coating composition highly viscous and adding a coagulation-preventing agent (e.g., a solid particle) have been somewhat effective, but these methods are not desirable because they are subject to restrictions in view of a filtrating step, a coating step and a size of the solid particles.
It has been proposed that a ferromagnetic fine particles be subjected to magnetic orientation in a coating direction in a strong magnetic field immediately after the coating composition is coated and before the solvent evaporates in order to improve orientation and magnetic characteristics of a ferromagnetic fine powder. However, even though a strong magnet is used, it normally takes 10 seconds or more to dry the solvent. Therefore, while a solvent is drying, orientation is disturbed due to the fluid property of the coating composition which allows free movement of the ferromagnetic fine powder. Accordingly, sometimes orientation disappears. Making a coating composition highly viscous has been proposed as a means of overcoming the above shortcoming. However, this causes another undesirable feature which is that the orientation of the ferromagnetic powder is poor in a magnetic field with normal magnet strength. If a strong magnetic field is used, the coated surface becomes considerably uneven, and noise and dropout increase. If drying speed is increased, the coated surface is disturbed by the wind used for drying.
In accordance with another method as described in Japanese Patent Application OPI No. 164436/82 (corresponding to U.S. patent application Ser. No. 364,346 filed Apr. 1, 1982) the support is coated with a coating composition having dispersed therein a compound capable of being polymerized by electron beam irradiation. Immediately after giving orientation of the ferromagnetic fine powder, the compound is polymerized by electron beam irradiation in the presence of a solvent so as to prevent the ferromagnetic fine powder from coagulation and disturbance of orientation. The solvent is then dried to prepare a coated layer having an even surface and having excellent dispersibility and orientation of the ferromagnetic fine powder.